KMS documentation uses removed feature flag @aws-cdk/aws-kms:defaultKeyPolicies

### Describe the issue

According to the V2 migration documentation, (almost) all feature flags were removed and @aws-cdk/aws-kms:defaultKeyPolicies was not an exception.

> Remove all feature flags from cdk.json. You can add one or more of the flags listed below, set to false, if your app relies on these specific AWS CDK v1 behaviors.

But @aws-cdk/aws-kms:defaultKeyPolicies is mentioned...
* 3 times in the 2.20.0 aws-cdk-lib.aws_kms module documentation.
* 1 time in the Key construct documentation.
* and possibly other places?

Strangely, the feature flag is still mentioned in https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/cx-api/lib/features.ts

Could it be the migration documentation that is wrong? That would possibly be worse.

### Links

https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_kms-readme.html
https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_kms.Key.html
https://docs.aws.amazon.com/cdk/v2/guide/migrating-v2.html#migrating-v2-v1-upgrade-cdk-json

Compare the V2 migration guide with the aws-cdk-lib.aws_kms module and Key construct documentation, then check the feature flag definition in packages/@aws-cdk/cx-api/lib/features.ts. Determine whether @aws-cdk/aws-kms:defaultKeyPolicies is still supported and update the documentation so its references consistently reflect that status.

Written by the indexing model from the issue text.